Quantum Molecular Dynamics Simulation Of Dielectric Function Of Solids And Its Temperature Dependence

The dielectric function is one of the fundamental thermophysical parameters and plays a crucial role in high-temperature thermal radiation. Recently, as the computing power enhances and the related algorithm improves, solving the equations of thermal radiation has become matured but how to obtain the high-temperature dielectric functions of solids remains to be the key issue. Limited by high-temperature oxidation and self-radiation, it is difficult to measure the dielectric functions of solids via direct experiments. On the other hand, the classical theoretical models, e.g., Drude and Lorentz, cannot effectively describe the temperature effect on dielectric functions and thus fail to predict the dielectric functions of solids at elevated temperatures. By now, the lack of high-temperature dielectric functions of solids has become the bottleneck for the study of thermal radiation at elevated temperatures.This thesis focus on investigating the underlying mechanism of temperature effect on dielectric functions of solids at the atomic scale. By combining the traditional first-principles method with quantum perturbation and lattice dynamics theory, the temperature effect is included to predict the high-temperature dielectric functions. The high-precision instruments of infrared variable angle spectroscopic ellipsometry(IR-VASE) and visible variable angle spectroscopic ellipsometry(V-VASE) are implemented to measure the dielectric functions of solids, in order to verify the feasibility of the improved method to predict the high-temperature dielectric function.Over the visible-ultraviolet spectrum, the optical absorption of semiconductors mainly arises from electronic interband transition. According to the Fermi’s gold rule, the dielectric function is intrinsically determined by electronic band structure, density of states and electron-photon interaction. This thesis applies the density functional perturbation theory(DFPT) and first-principles molecular dynamics(FPMD) to predict finite temperature dielectric functions of element semiconductor Ge. As the study indicates, both of the DFPT and FPMD methods can predict the temperature effect on absorption peak, such as redshift, reduced amplitude and broadened width. The redshift can be interpreted by the reduced band gap, which requires lower photon’s energy to excite interband transition. In contrast, the calculated results of FPMD approach demonstrate better agreement with literature experiments than those of DFPT method at 825 K. On the other hand, the FPMD method is applied to investigate the temperature dependence of dielectric functions of semiconductors Si C, Six Ge1-x and Si3 Al Asx P1-x. As the results indicate, the FPMD method can predict the finite temperature on dielectric function, thus verifying its feasibility to predict the high-temperature dielectric functions of semiconductors.With its ionic character, the infrared optical absorption of metal oxides mainly arises from the coupling of lattice vibration with incident optical electric field. Based on the linear response theory, the FPMD method is applied to calculate the time-correlated dipole moment and predict the infrared dielectric functions of Mg O at elevated temperatures. The FPMD method predicts the sharp absorption peak of dielectric function around 400 cm-1, and concludes that the amplitude damps, the position shifts to longer wavelength and the width broadens as temperature increases, which is in good agreement with IR-VASE experiments. The phenomena are caused by the coupling among phonons, which induces the reduced lifetime, lowered vibration frequency and damped phonon-photon interaction. Since it computes the interatomic potential by solving the Schrodinger’s equation, the FPMD method can predict the infrared absorption spectrum of Mg O at 1950 K and demonstrate good agreement with literature experiments. The FPMD method is also applied to predict the dielectric function and radiative properties of Al2O3 around the melting temperature in the spectral range 4-12 μm. As the lattice vibration enhances, the infrared absorption peak damps and its position shifts to longer wavelength.Differs from bulk material, there exist dominant quantum-size effect, singular electronic band structure and density of states in low-dimensional materials. The FPMD method is applied to study the dielectric function and its temperature dependence, and analyze the quantum-size effect. As temperature increases, the absorption peak of isolated zigzag(5,0) nanotube enhances, which is caused by the disturbed singular density of states. For the bundled nanotube, the separation distance is about 2.8-3.2 ? and as the separation distance decreases, the non-local effect strengthens. When the intertube distance is larger than 20 ?, the intertube interaction can be neglected. As temperature increases, the absorption peaks of two-dimensional graphene and silicene shift to lower energy and the amplitudes enhance. The normalized optical constants of mono-layer graphene demonstrate good agreement with V-VASE ellipsometry experiment. Due to its large bandgap, the absorption peak of arsenene demonstrates reduced amplitude as temperature increases, which is contrary to that of graphene and silicene.